A heat exchanger such as a synthesis gas waste heat boiler is subject to a number of special conditions, which are difficult to account for by combination in one design.
These conditions are related to the pressure, temperature, nitriding, hydrogen attack and stress corrosion.
The ammonia synthesis gas will typically be at a pressure of 120-220 bar. The boiling water will typically be at low (5-15 bar), medium (30-50 bar) or high pressure (90-130 bar). Separation walls between synthesis gas and boiling water must be designed for the highest pressure difference of the two fluids. In shell and tube heat exchangers this will normally result in a very thick tube sheets usually with a thickness of 300-450 mm.
The ammonia synthesis gas can be between 380° C. and 500° C. at the inlet to the boiler and between 200° C. and 380° C. at the outlet. The boiling water can be between 150° C. and 330° C., depending on the steam pressure.
Synthesis gas waste heat boilers are often designed as u-tube exchangers with a very thick tube sheet. The thick tube sheet will obtain a metal temperature which is close to the gas temperature of the sheet penetrating tubes. In case of u-tubes, this will in known art imply that the inlet tube area will be hot where as the outlet tube area will be cold. High thermal induced stresses are therefore a risk, if the temperature difference between inlet and outlet gas is too high. In case of low and medium pressure steam production is it desirable if a temperature difference of 200° C. to 300° C. could be acceptable. It has however in know art shown difficult or impossible to design a u-tube waste heat boiler for such a big temperature difference.
Nitriding is a materials attack caused by the ammonia content of the synthesis gas. The severity of nitriding depends on the metal alloy and the metal temperature. Low alloy steels are attacked unacceptably at 380° C. Stainless steel can be used to 450° C. or higher and Iconell will not be severely attacked even at 500° C. The inlet-tube area of the tube sheet in a U-tube boiler will often be hotter than 420° C. The materials, in contact with the synthesis gas must therefore be high alloy. A surface protection by cladding or lining will be required on the gas side of the tube sheet and through the inlet-hole surface.
Hydrogen attack will cause embrittlement in materials when exposed to hydrogen containing gasses. The important parameters are the hydrogen partial pressure, the temperature and the alloying elements of the steel. 2% Cr and 1% Mo steel alloy will typically be required by industrial synthesis gas composition, pressure and temperature.
Stress corrosion is a risk for the materials in connection with the water. This kind of corrosion is however not critical by ferritic materials, whereas austenitic materials are sensitive to this kind of attack. The typical synthesis gas waste heat boiler is a U-tube heat exchanger with synthesis gas on the tube side and water/steam on the shell side. The tube sheet is very thick. The inlet side of the tube sheet is protected by Inconell cladding. If the tubes are welded to the gas side of the tube sheet, the tubes must be lined on the inner surface with Inconell all the way through the tube sheet. If the tubes are welded to the waterside of the tube sheet, the inlet holes of the tube sheet must be protected by an Inconell lining.
Synthesis gas waste heat boilers often fail due to cracks caused by one or a combination of the described mechanical and/or corrosion phenomena. The most severe conditions among these are concentrated around the inlet tube holes. That is due to the high temperature, the temperature difference between inlet and outlet tubes, stress corrosion, hydrogen build up between materials of different composition, nitriding and hydrogen attack. Another aspect of the Synthesis Gas boiler is the pressure drop of the synthesis gas through the exchanger, which have to be kept low due to considerations of power/energy consumption of the synthesis gas compressor.
In U.S. Pat. No. 3,568,764 a u-tube heat exchanger is disclosed where a baffle is provided adjacent to the outlet side of the tube sheet of the multiple tube pass heat exchanger. A portion of the cold input fluid is passed between the baffle and the tube sheet, rather than through the tubes, so that the tube sheet is maintained at a substantially uniform and cold temperature. Ferrules pass the heated outlet gas portions from the tubes to the outlet chamber of the channel. The heat exchange efficiency is however lowered due to the portion of input fluid which by-passes the heat exchange tubes. The heating fluid is on the shell side of the exchanger, which is contrary to present invention where the cooling fluid is on shell side.
In EP 0860673 a solution to the above problems is disclosed by a fire tube heat exchanger with a plurality of heat exchanging tubes, wherein the heat exchanging tubes are in form of a double tube with an outer tube closed at one end and an open ended inner tube spaced apart from the outer tube, adapted to exchange heat between a hot gas on tube side of the outer tube and a fluid on shell side of the tube. Though solving the above mentioned problems, this solution has however a considerable pressure drop on tube side compared to an U-tube exchanger, which renders the solution more expensive due to expenses in relation to increased heat exchange surface for a given pressure drop.